Valid Limited Server Access (full access but only one server at a time)

What about toning it down a bit:
Players can join as many servers as they want, but non-paying players can only have claim blocks on one server at a time, and a command to "transfer" your claim blocks to another server put in place.
Alternatively, a "free first month", with clear warnings, during which players can use any of the servers, but after one month from the date they first log into their *second* MYM server we are limited to just one server at a time unless we pay a subscription.